1. Field of the Invention
The present invention relates to a frequency demodulation circuit for detecting a modulated signal from a FM (frequency modulation) carrier signal.
2. Prior Art
Frequency modulation (hereinafter referred to as "FM") has a higher S/N than an amplitude modulated wave with respect to external noises and it is a modulation system commonly used for transmitting information by means of television voice signals, stereophonic FM broadcasting and the like.
As a FM detector for detecting a modulated signal from a FM-modulated carrier signal, there is commonly used a ratio detector, Foster-Seely detector and etc. but with the development of semiconductor technology, Quadrature detectors, PLL-type detectors or the like have come to be incorporated into receivers.
FIG. 6 is a block diagram of a Quadrature detector of the type mentioned above. In this figure, reference numeral 1 designates a 90 phase-shifting circuit having a FM carrier signal inputted therein, reference numeral 2 designates a band-pass filter (BPF) which has been adjusted to make the FM carrier frequency its center frequency and reference numeral 3 designates a multiplier which detects the phase difference between the above-mentioned FM carrier signal and the output of BPF 2 to thereby detect a FM signal. Then the detected signal is outputted as a voice signal through a low-pass filter (LPF) 4.
Assuming that the angular frequency of the FM carrier signal is .omega., the BPF 2 generates a signal of lead phasewith respect to the center frequency f.sub.0 (.omega..sub.0) thereof when the relationship of .omega..sub.0 &gt;.omega. is established while it generates a signal of lag phase when the relationship of .omega..sub.0 &lt;.omega. is established.
Accordingly, as shown in FIG. 6B, when the product of the signal waveform of a FM carrier signal .omega.+90.degree. inputted to the BPF 2 and the output waveform .omega. of the BPF 2 only when it is positive is detected, the detection output in a period in which both of the FM carrier signal .omega.+90.degree. and the output waveform w are positive is detected. The average voltage of this output changes around .omega.=.omega..sub.0. Accordingly, the modulation signal component of the FM carrier signal can be detected by integrating this output with the LPF 4.
FIG. 7 is a schematic block diagram of a PLL-type FM demodulator wherein reference numeral 5 designates a phase comparator, reference numeral 6 designates a low-pass filter (LPF) and reference numeral 7 designates a voltage-controlled oscillator (VCO) the oscillation frequency of which is controlled by the output of the LPF 6.
As is well known, this circuit is applied with a feedback control so that the VOC 7 is locked to the frequency of the inputted FM carrier signal and a difference signal corresponding to the change in the center frequency of the VCO 7 and the frequency of the FM carrier signal is taken out of the LPF 7.
Every FM demodulation circuit of the type described above uses a phase variable circuit comprising a BPF, substitutes the difference between the FM carrier frequency fo for a phase deviation and then converts it into a voltage amplitude by using a phase comparator thereby detecting a FM signal wave.
Accordingly, it is necessary that the reference frequency fo be made to coincide with the center frequency of the FM carrier signal and the demodulation characteristics of the detector depend much on the accuracy of coincidence.
For this reason, the variation of f0 taking place by the nonconformity of the values of the constituent elements (such as a coil L and a capacitor C) of a variable phase device has hitherto been corrected by adjusting the value of such coil or capacitor. However, such adjustment requires not only a quite high level of accuracy of itself but also the preciseness of temperature range, secular change and the constituent elements of the device, which results in an increase in the cost of the entire system.
Further, where the above-described FM demodulation system is used for the voice signal demodulation circuit of a television receiver, if the television is of the NTSC system, the voice carrier frequency is in the order of 4.5 MHz while when, in Europe, it is of the B/G system, the voice carrier frequency is in the order of 5.5 MHz, when it is of the D/K system, that frequency is in the order of 6.5 MHz and when it is of the I system, the carrier frequency of the voice FM signal is in the order of 6.0 MHz, depending on the district where the television is used.
Accordingly, there has arisen the problem that these television receivers have to be provided with FM demodulation circuits having different center frequencies fo and so the number of parts and the number of portions to be adjusted increase inevitably thereby reducing the production efficiency.